Torque transmitting systems are widely employed in automobile transmissions to provide selective, relative rotation between components of the transmission. As is well known to the art, one widely accepted form of an automatic, vehicular transmission employs compound planetary gear sets that utilize three clutch assemblies and two braking bands to preclude relative rotation between selected components in order to obtain the desired function of the compound planetary gear sets. The operator selects the driving range from the neutral, forward (either the standard drive, the "Intermediate" or the "Lo" forward selections) or reverse, and the transmission automatically changes gear ratios in relation to the vehicle speed and the engine torque input, as permitted within the range selected. Vehicle speed and engine torque signals are constantly fed to the transmission in a manner well known to the art in order to provide the proper gear ratio for maximum efficiency and performance at all throttle openings.
A planetary gear train consists of a center, or sun, gear, an internal gear and a planetary carrier assembly which includes and supports the smaller planetary gears, or pinions. When the sun gear is held stationary and power is applied to the internal gear, the planetary gears rotate in response to the power applied to the internal gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the internal gear is being rotated.
When any two members of the planetary gear train rotate in the same direction and at the same speed, the third member is forced to turn at the same speed. For example, when the sun gear and the internal gear rotate in the same direction, and at the same speed, the planetary gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the internal gear, the planetary gears act as idlers. In that way the driven member is rotated in the opposite direction as the drive member. Thus, when the reverse drive range is selected, a brake band assembly is actuated frictionally to engage the carrier assembly, and restrain it against rotation, so that torque applied to the sun gear will turn the internal gear in the opposite direction in order to reverse the rotational direction of the drive wheels, and thereby reverse the direction of the vehicle itself. The friction band assemblies are normally operated by servo mechanisms, many varieties of which are known to the art, but the present invention does not relate to servo mechanisms, and they will not be further described herein.
It should be appreciated that a second friction applying band assembly may also be employed when the engine compression, acting through the transmission, is being employed to effect a braking action. To understand this operation it is desirable to know that in a compound planetary gear set, multiple planetary gear sets may be employed, and adjacent planetary gear sets may utilize sun gears fabricated in one piece. A sprag assembly is frequently employed selectively to preclude the common sun gears from rotating in one direction.
Adjacent planetary gear sets also generally connect the carrier of the first set to the internal gear of the second set. To make the two planetary gear sets effective a roller clutch assembly is generally employed to hold the carrier of the second set against rotation in at least one direction.
To provide a means of connecting and disconnecting the power output from the converter to the transmission gear train, a clutch assembly is generally employed. Typically a clutch assembly includes a clutch housing which is splined to the input shaft. A series of torque plates are connected, as by tangs, to the clutch housing, and a second series of torque plates are connected, also by tangs, to a clutch hub member. An actuating piston is hydraulically operated frictionally to lock the torque plates together, and a release spring is employed to retract the piston when the hydraulic pressure is released. By effecting a spline connection between the main transmission shaft and the clutch hub member, whenever hydraulic pressure is supplied to the clutch assembly the input shaft directly rotates the main transmission shaft. When the hydraulic pressure is released, the clutch assembly disengages the aforesaid drive connection, and the transmission is in neutral.
A similar clutch arrangement may also be employed selectively to connect the outer race of the sprag to the transmission housing. When the outer race of the sprag is so connected to the housing, the sprag is effective in securing the sun gear connected to the sprag against rotation, and the power output from the converter is received by the transmission output shaft at the gear reduction ratio associated with "second" gear.
A third such clutch arrangement is employed to lock the pinions of the adjacent planetary gear set together so that they act as wedges to allow the two adjacent planetary gear sets to rotate as one unit. In this arrangement the power output from the converter is received by the transmission output shaft in what is designated as "third" gear.
Actuation of the first and third described clutch arrangements is generally effected when the operator selects the "reverse" range of operation.
Piston return springs are required and customarily employed to force the piston back into it's bore after piston apply pressure has been removed. These springs are usually located around the periphery of the clutch pack and react against either the piston or the first reaction plate to push the piston back. Inasmuch as these springs are on the same side as the clutch plates, they are located radially either inside or outside of the cylindrical area taken up by the clutch plates.
When the piston is applied, there is a bending moment generated between the piston apply force and the spring return force. This is because the springs are not located in the center of the piston. This bending moment causes deformation of the piston or the first reaction plate, resulting in non-uniform loading of the friction plates.
This uneven radial loading on the friction plates results in uneven unit and thermal loading, thereby causing friction material of bonding failure to occur earlier than what would occur if the loading were uniform.